DESCRIPTION: Cellular mechanisms regulating tone in the chronically hypoxic pulmonary circulation are poorly defined. A growing body of evidence suggests that altered endothelial control of pulmonary vascular tone plays a critical role in pulmonary hypertension (PHT), and that a key mediator in the process i endothelin-1 (ET-1). The investigators' data suggest that ET-1 is acting by stimulating Ca2+ influx through Ca2+ channels other than the L-type channel, possibly low threshold voltage-gated (T-type) Ca2+ channels. However, the mechanisms linking ET-1, PA smooth muscle cell (SMC) membrane potential and Ca2+ influx in the hypertensive pulmonary circulation at the cellular level have not been defined. Additionally, in question is the effect of chronic hypoxia on PA SMC ion channel expression and regulation. While it is known tha chronic hypoxic PHT reduces macroscopic K+ currents in PA SMCs, which might render the cells more depolarized, the full effects of chronic hypoxia on PA SMC ion channels and the mechanisms through which ion channel activation is altered are unknown. As most studies have utilized chronically hypoxic animals it is also uncertain whether hypoxia, per se, alters PA SMC ion channel function, or whether hemodynamic stress and mediators, such as ET-1, play a role. Also unknown is whether the predominant "normal" population of PA SMCs differentiate into a "hypertensive" phenotype, or conversely, whether a unique sub-population of cells constitutively expressing this phenotype expand within the remodeling PA wall. The investigators will test the hypotheses that in chronic pulmonary hypertension: 1) ET-1 causes depolarization of PA SMCs via inhibition of delayed rectifier K+ channels and activation of PA SMC non-selective cation channels. 2) Novel routes of Ca2+ entry in response to ET-1 and hypoxia develop, including low threshold voltage operated Ca2+ channels. 3) Either hypoxia, mechanical stress and/or ET-1 cause differentiation of "normal" PA SMCs into a hypertensive phenotype (i.e.: decreased K+ channels and novel Ca2+ channels). As a corollary to Aim 3, uniqu populations of PA SMCs isolated from pulmonary hypertensive calves which stabl express hypoxia-sensitive or hypoxia-resistant growth will be compared, testin if constitutive alterations in channel expression and [Ca2+ ]i regulation are present.